1. Field of the Invention
This invention relates to impellers of centrifugal pumps used in industrial applications. More specifically, this invention relates to impellers made of very hard materials which are conventionally structured with a lead babbitt to receive a drive shaft. The present invention provides structures and methods for eliminating the babbitt to provide an impeller that is environmentally safe and less expensive to produce.
2. Description of Related Art
Certain industrial processes involve the pumping of extremely abrasive materials. Such industrial processes include, for example, raw sewage treatment and mining and dredging where the slurries being pumped contain highly abrasive solids. While all pumps that process slurries are eventually subject to wear and degradation, those pumps that are used to process highly abrasive slurries are susceptible to faster and more significant degradation.
Responsive to the wear imposed by processing such highly abrasive slurries, pump impellers have been made of more durable material to withstand the wear. Many such impellers, for example, are made from very hard metals selected to be harder than the most common and abrasive grit particle, which is silica sand. The materials are generally selected, therefore, to have a hardness greater than 570 Bhn on the Brinell Hardness scale, or the equivalent thereof. Materials having a hardness greater than 570 Bhn include Ni-Hard and Hi-Chrome. The use of hard materials in the formation of pump impellers significantly improves the life of the impeller, but also imposes difficulties in the manufacture of the impeller.
Pump impellers are typically rotated within a pump casing by connection to the drive shaft of a motor. Impellers are generally formed with a central cavity or opening into or through which the terminal end of the drive shaft extends. The exact design and construction of the connection of the impeller to the drive shaft varies widely between types and models of impellers. Impellers that are made of softer metals may typically be machined to form a central cavity that will accommodate the end of the drive shaft. However, impellers that are made of very hard materials (i.e., greater than 570 Bhn), are very difficult to machine and, therefore, present a problem with fitting the drive shaft to the impeller.
It has been the conventional practice with very hard material impellers to form a babbitt in the central cavity of the impeller to receive the terminal end of the drive shaft. The babbitting is typically lead and the softness of the lead babbitt allows it to conform to the drive shaft to provide comprehensive contact between the babbitt and the drive shaft. The babbitt may be formed with a given configuration to accommodate the drive shaft.
In known casting techniques, the impeller is made in a mold which is shaped to produce a central cavity in the impeller. The central cavity of the casting is of imprecise dimension and finish which is permissible since the babbitt formed in the central cavity compensates for any dimensional or finishing imprecisions. Once the molten material of the impeller has hardened and the casting is removed from the mold, the central cavity is ready for the formation of the babbitt. The center of the cavity is determined and a post-like implement or mandrel is positioned at the center of the cavity. Molten lead is then poured into the cavity and around the mandrel. When the lead has hardened, the mandrel is removed. The babbitt may be formed with a particular shape that is dictated, at least in part, by the machining of the end of the drive shaft.
The need to use a lead babbitt, necessitated by the extreme hardness of the impeller material, results in significant additional labor which increases the time and cost of manufacturing hard material impellers. More importantly, however, is the fact that lead babbitts cannot be used in many applications because the lead seeps into and contaminates the water being processed. Also, formation of the lead babbitt is a very toxic and dangerous process and is very costly as a result.
An alternative method of mounting very hard material impellers to drive shafts is to provide a soft metal insert into the center of the impeller mold prior to pouring the molten material to form the impeller. The soft metal insert can be configured to provide contact with the impeller and to accommodate the drive shaft, but is also machined to receive the drive shaft. Soft inserts are used when the type of fluid being pumped is incompatible with the lead of a babbitt. The use of soft inserts, however, also represents added cost and labor to the manufacture of the impeller because of the machining required to manufacture the insert and the additional machining required to form the insert to the drive shaft.
Thus, it would be advantageous in the art to provide means for producing impellers from hard material which eliminates the need for a lead babbitt or any other type of insert to accommodate the drive shaft and which eliminates the need to machine the impeller or drive shaft, significantly reduces manufacturing costs, simplifies manufacture and provides a more environmentally safe impeller of very hard material.